1. Field of the Invention
The present invention relates to the art of simulation and more particularly relates to devices which simulate vision loss which, for example, occurs when a subject is subjected to sustained large acceleration forces.
2. Brief Description of Prior Art
When a subject experiences a large drop in blood pressure in his head, particularly at the temporal level, a gradual loss of vision usually follows. For example, during maneuvers pilots of tactical aircraft are subjected frequently to high G-loading which causes such a blood pressure drop. At sufficient magnitudes of upward acceleration, e.g., during a pull-out from a dive, a pilot's vision will gradually fade and actually disappear before the pilot passes out. Such complete vision loss, even though consciousness persists, is often referred to as "black out".
Under G-loading, vision loss proceeds according to a schedule which follows a fairly well defined pattern, although there are differences in the subjective visual perceptions reported by subjects. The fading begins as a gradual loss of contrast throughout the subject's field of view. At sufficiently high G's, a more severe peripheral fading, or dimming, begins at the edges of the subject's field of view and proceeds inwardly toward the center. Subjects report that this is like looking through a tunnel of diminishing diameter, hence the term "tunnel vision" is used to describe the perception.
The dimming occurs as an increasing loss of contrast caused by a gradual increase in the threshold of perception. First, faint lights are lost from view in the dimming area, and then gradually brighter and brighter lights disappear in the effected area until vision is lost completely. The onset effects have been reported as a perception of "graying" of the field of view.
In addition, subjects have reported a "misting" or "veiling" effect at the edges of the advancing area of fading, as if that portion was being viewed through a veil. Some subjects have reported that this effect, as well as the greying effect, advances from the periphery first in "spikes" or "filaments" followed by a more uniform area of affectation.
It is believed that the reason the dimming proceeds in the pattern just described is that the drop of pressure causes blood to drain from the retinal arteries in a sequence starting first with capillaries and proceeding to larger and larger arterial branches toward trunk arteries which eminate from the center of the rear of the eye. This decrease in blood supply occasions oxygen starvation which causes vision loss. Hence the filament-like advancement of the effects from the periphery toward the center.
Another visual disturbance, associated with fore and aft accelerations of a subject, is a reported blurring of vision. Photographic studies of an eyeball under high decelleration show that a film of lacrimal fluid issues from the lacrimal glands and collects on the cornea under such conditions. It is suspected that this causes the reported blurring.
Also experienced, under extreme downward acceleration, is a phenomenon known as "red-out" among pilots. It is experienced as the perception of a red screen which advances from the bottom of the subject's field of view. During such accelerations the entire cranial area becomes engorged with blood, including the area around the lower eyelid. It is believed that the lower eyelid swells and is forced upward to infringe on the area of the pupil. Thus the occurrence of "red out".
Experienced pilots utilize these cues in the operation of aircraft. In particular, pilots learn to judge how close to black-out they are by the extent to which the peripheral onset effects have advanced. This is of particular importance to pilots in conditions where visual contact of an adversary must be maintained.
It is thus desirable in the simulation art to give the pilot the advantage of experiencing these important cues while piloting a simulator, in order to train the pilot in their utilization. To this end, Carl E. Hoyt and Timothy E. Hale invented a device and method for simulating visual black-out, disclosed in U.S. Pat. No. 3,942,270 which issued Mar. 9, 1976. In one embodiment of that invention, a special visor having a non-uniform layer of liquid crystal provides a variable opacity screen. The liquid crystal is sandwiched between a pair of curved, planar transparent electrodes which define a container for the liquid crystal. This container is likewise of curved, planar configuration, but has a thickness which smoothly varies from a relatively thin center point to a thicker periphery. Exciting the liquid crystal with a voltage causes the crystal to turn opaque. However, the voltage level at which the opacity is produced varies according to the thickness of the layer. Thus, in the configuration just described, as the voltage is increased opacity advances from the periphery of the visor to the center point. In this manner the advancing fading of onset blackout visual effects may be simulated.
In another embodiment, a liquid crystal visor is provided with a row of electrodes in the form of horizontal strips opposite a planar electrode of opposite polarity. The strip electrodes are sequentially excited from, e.g., bottom to top to produce an effect which advances upwardly. Such a device can be used with polarizing liquid crystals in conjunction with polarized red light to produce a red screen which advances upwardly to simulate "red-out".
The invention disclosed in U.S. Pat. No. 3,942,270 represents an important advance in the art of simulation. However, it suffers several significant shortcomings. Probably the most serious is the fact that the simulated effects are produced relative to a fixed, central, single "line of sight". Thus, while that line moves as the subject moves his head, it does not move as the subject moves his eyes, as do the actual effects. This detracts from the realism of the device.
Further, limited flexibility is offered in the configuration of the screening effect produced. Indeed, the preferred embodiment contemplates simply a transparent circular central area bounded by an area of opacity. There is, for example, no provision for (nor mention of) the filamenting effects noted earlier.
The present invention provides a vision loss simulator which successfully overcomes these shortcomings and provides greatly expanded flexibility in producing the visual effects associated with the onset of blackout. In fact, the present invention is capable of being used as a research tool, for example to explore empirically which are the most significant cues which pilots rely on in the operation of their craft.